Ancient Cooking Stones Help Reconstruct Earth’s Magnetic History

The stones achieve temperatures as high as 1,100ºC. Credit: Gillian Turner

Cooking stones that line Maori steam ovens in New Zealand are helping scientists study the history of the Earth’s magnetic field. The cooking process generates so much heat that the magnetic minerals in the stones will realign themselves with the direction of the current field.

The scientists presented their findings at the fall meeting of the American Geophysical Union (AGU). More archaeological searches are underway in order to find older ovens. They are called hangi. Abandoned stones at these locations could help elucidate the history of Earth’s magnetic Field, going back hundreds of years.

The experimental hangi proved the principle. Credit: Gillian Turner

There is a good record of palaeomagnetic data from across the world, but there is a gap in the southwest Pacific. In order to complete global models, more data is needed.

The project involves retrieving information about the Earth’s magnetic field and its changes, going back over the past 10,000 years. For data on the last few centuries, pottery is usually used. When these objects are fired, the minerals in their clay are heated above the Curie temperature and they are demagnetized.

As the pots cool down, the minerals become magnetized again in the direction of the prevalent field. The strength of the magnetization is directly related to the strength of the field. The first settlers of New Zealand, the Maori, 700 to 800 years ago, didn’t use pottery. However, the Maori have a cooking tradition of using a steam oven. The pits are dug in the ground and very hot stones are placed inside, covered with baskets of food and layers of fern fronds soaked in water. The oven is topped off with soil and left to cook for several hours. Gillian Turner, lead author, and her team experimented with modern-day hangi to see if they could achieve the necessary Curie temperatures to reset their magnetization. By putting thermocouples in the stones, the team showed that the stones got as high as 1,100ºC. At this temperature, rock-forming minerals start to become plastic.

A compass was placed upon the cooled hangi stones and it was used to determine that re-magnetization had occurred. Hangi stones were carefully chosen, and the most popular types were from andesite boulders found in the Central North Island. These volcanic boulders were chosen because they don’t crack or shatter in the fire. They also behave best from a magnetic point of view, since they contain a higher concentration of magnetite.

Hangi stones are likely to lead back to the 1200s. For deeper time magnetic data, they will need to find other sources.